Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove inner panel, fan and burner
Unscrew sump connection on condensate trap
Remove 4x screws to remove burner box assy.

Observation:

Inspect burner box for debris build-up.
Check seals around burner box are in good condition.
With the aid of a mirror inspect the rear of the heat exchanger flueways for blockages or restrictions.
Note:
Before reassembly check integrity of all disturbed gaskets and seals, replace as necessary.